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Ion back-flow gating in a micromegas device

Abstract : As several research and development programs on Gaseous Photomultipliers are running on, the main aging factor on the photocathode, in particular for bi-alkali photocathodes, is the ion back-flow. This effect has also a major impact on the behaviour of Time Projecting Chamber in terms of electron drift perturbations. This paper presents a new device that has been tested with a Micromegas in order to reduce the ion back-flow in this type of detector. Instead of using a single mesh, as it is the case in a classical use of this micropattern detector, a double-mesh is installed. When the good conditions of electric field are applied, the device is transparent to electrons but gates the ions. Several measurements of ion back-flow ratio have been performed using a high power Xenon lamp to create photo-electrons on a Ni photocathode. The currents on the different electrodes are then measured with a pico-ammeter and the ion back-flow ratio is given by the ratio I_c/I_a (where I_c and I_a are respectively the cathode and the anode currents). For a classical mode of working (single mesh mode) the Micromegas ion back-flow is about few percent and is proportional to the field ratio. We have demonstrated that it is possible to reach a back-flow ratio close to few 10^−6 in a double mesh mode, for a gain of several thousands, depending on the gas mixture and the electric fields ratio between the different electrodes.
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Submitted on : Monday, March 15, 2010 - 4:59:11 PM
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F. Jeanneau, M. Kebbiri, V. Lepeltier. Ion back-flow gating in a micromegas device. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Elsevier, 2010, 623, pp.94-96. ⟨10.1016/j.nima.2010.02.159⟩. ⟨in2p3-00464017⟩



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